In electrical field, there is an ideal topology circuit structure, i.e. the three-phase Vienna structure. Circuits of such structure have advantages such as small input current ripple, less device requirements, simple control and low cost, and thus are widely applied in the case of three-phase alternating current high power input.
A circuit diagram of the three-phase Vienna circuit structure is shown in FIG. 1. The circuit has circuits for three phases. The connection structure of the circuit for one phase is: a inductor L1 is connected to the anode of a diode D1 and connected to the cathode of a diode D4; the cathode of the diode D1 is connected to the positive electrode of a capacitor C1; the anode of the diode D4 is connected to the negative electrode of a capacitor C2; the negative electrode of the capacitor C1 is connected to the positive electrode of the capacitor C2; a connection point connecting the inductor L1, the anode of the diode D1 and the cathode of the diode D4 is connected to the drain of a switch transistor Q1; the source of the transistor Q1 is connected to the connection point of the capacitor C1 and the capacitor C2.
The devices in each of circuits for the other two phases of the three-phase Vienna circuit structure are connected in a way the same as that in the above mentioned circuit for the one phase.
In the circuit of the above-mentioned topology structure, the voltage difference between the source and the drain of the switch transistor Q1 is large. Moreover, in practical application of the circuit of such topology structure, the drain of the switch transistor Q1 is connected to each of the anode of the diode D1 and the cathode of the diode the D4 by a long wire, leading to parasitic inductance existing between the drain of the switch transistor Q1 and the anode of the diode D1 and between the drain of the switch transistor Q1 and the cathode of the diode D4. Therefore a very high voltage spike may be generated at the moment when the switching diode Q1 is turned off.
In order to avoid the problem of the very large voltage spike generated at the moment when the switching diode Q1 is turned off, generally an inductor R1 and a capacitor C3 are connected between the drain and the source of the switch transistor Q1. The inductor R1 and the capacitor C3 are connected in series with each other and then connected in parallel to the switch transistor Q1. This kind of circuit is generally referred to as a RC snubber circuit. In the operation of the circuit, when the switch transistor Q1 is turned off, the voltage spike generated at that moment charges the capacitor C3; when the switch transistor Q1 is turned on, the capacitor C3 discharges and electric charges released are consumed by the inductor R1.
The snubber circuit of such RC structure still has the following disadvantages.
In the case where the capacitance of the capacitor C3 is small, while the voltage spike generated at the moment when the switch transistor Q1 is turned off is too large, the RC snubber circuit can not efficiently depress the voltage spike.
In the case where the capacitance of the capacitor C3 is large enough, a large voltage spike generated at the moment when the switch transistor Q1 is turned off may be absorbed by the capacitor C3. However, when the switch transistor Q1 is turned on again, the capacitor C3 discharges. At this point, since the capacitance of the capacitor C3 is large enough, the quantity of the electric charges released by the capacitor C3 is large. If the released electric charges cannot be consumed by the inductor R1, the residual electric charges may influence the output junction capacitance of the switch transistor Q1, increase the loss of the switch transistor, and affect the efficiency of the entire circuit.
Therefore, it can be seen that, in the existing RC snubber circuit, the problems of effective depression to the voltage spike generated when the switch transistor Q1 is turned off and the voltage stress difference when the switch transistor Q1 is turned on cannot be solved simultaneously.